Discharge gap structure for lighting arresters

ABSTRACT

Discharging gap structures are disclosed including two discharge electrodes disposed in divergently spaced, opposite relationship. End portions of the electrodes define a minimum gap size and each have a radius of curvature less than the minimum gap size. The radius of curvature is not greater than 0.3 of the minimum gap size and smaller than that of the other end portions of the electrodes defining a larger gap size. An arc-extinguishing plate may be positioned 15 mm or more from the electrodes with a space adjacent the electrodes filled with 0.1 gram or more of SF6 gas.

atent 1 Nitta et al. [4 1 Apr. 10, 1973 DISCHARGE GAP STRUCTURE FOR I I LIGHTING ARRESTERS [56] References Cited [75] Inventors: Tohei Nitta; Yoshikazu Shibuya; UN STATESIPATENTS I Naoya Yamada; Yukio Fujiwara, all I r of Amagasaki Japan 3,361,923 1/1968 Osterhout ..315/36 x [73] Assignee: Mitsubishi Denki Kabushiki Kaisha, Prim r Examiner-Palmer C. Demeo Tokyo, Japan Attorney-Robert E. Burns and Emmanuel J. Lobato 22 F'l i I I red Mar 1, 1971 ABSTRACT I [21] 1 Appl' l20028 Discharging gap structures are disclosed including two I discharge electrodes disposed in divergently spaced, [30] Foreign Application Priority Data opposite relationship. End portions of the electrodes define a minimum gap size and each have a radius of I m japan curvature less than the minimum gap size. The radius 1970 l 6 of curvature is not greater than 0.3 of the minimum ar. apan gap Size and Smaller than tha of he t e d po I tions of the electrodes defining a larger gap size. An [52]- US. Cl ..3l3/214, 313/217, 3l3/DlG. 5, arc xtinguishing plate may be positioned 15 mm or Int Cl more from the electrodes with a space adjacent the J l t d d 01 s .[58] Field of Search... ..3l3/DIG. 5, 155, 6 ac to es 1 6 W1 gram or mQre o 6 gas 7 Claims, 15 Draviing Figures PATENIEDAPR 1 01975 SHEET 1 [1F 2 V1 Q0 I v FIG. 3

PRESSURE OF SF =2ABS.ATM. ARC CURRENT 2kA ARC CURRENT GAP SIZE Irnm GAP SIZE IN mm PRESSURE OF SP IN ABS. ATM.

PAIENIEDAFR 1 01975 SHEET 2 0F 2 BE ND DISTANCE TWEEN ELECTRODE A WALL' IN mm DISCHARGE GAP STRUCTURE FOR LIGHTING ARRESTERS BACKGROUND OF THE INVENTION gaps of lightning arrester devices but the attempts have i been not much successful. This is because unlike circult interrupters, lightning arrester devices have the performance determined by a ratio between a magnitude of the dielectric strength after the interruption of a discharge current and'a magnitude of a discharge voltagebefore the interruption of that discharge cur- V type of discharging gap structures including a plurality of elongated electrodes round at both ends and edges and disposed in'a divergent, opposite relationship to form discharging gaps narrower at one end than at the other ends therebetween, and magnetic means for establishing a magnetic field substantially perpendicular to the general plane of the electrodes to rapidly drive the electric arcs developed across the narrower ends of the gaps toward the broader ends of the gaps. If SF gas high in dielectric strength is utilized as an arc quenching medium, with the discharging gap structures of the type above described then the minimum discharging gap across which an electric arc is initially developed between the adjacent electrodes is inevitably decreased in size. This is remarkable particularly for gaps disposed in the atmosphere of high pressure SF gas. The use of a small gap size has given rise to a serious problem in magnetically driving electric arcs. Generally, upon magnetically driving electric arcs, some interval of time lapses until the electric arc is initiated to be moved after the application of a magneticfield thereto. That interval of time tends to be longer for SF gas. This has resulted in a disadvantage that the interrupting performance is not good. In addition, for higher discharge currents it has become difficult to magnetically drive the resulting electric arcs. Thus SF filled arrester devices including the discharging gap structure of the type above described are disadvantageous in that a heavy duty can not be born.

SUMMARY OF THE INVENTION the abovementioned disadvantages of the conventional devices are eliminated.

The invention accomplishes this object by the provision of a lighting arrester device comprising an electrically negative gas forming an arc-extinguishing medium, and a pair of discharge electrodes disposed in divergent, opposite relationship to form a discharging gap therebetween, characterized in that each of the electrodes has a discharge initiating portion shaped such that radius of curvature in the cross section thereof is smaller than a size of that portion of the discharging gap defined by the discharge initiating portions of the electrodes.

Preferably, the minimum radius of curvature should not be greater than 0.3 of the size of the discharging gap portion defined by the discharge initiating portions of the electrodes, and those portions'of the discharge electrodes opposing to each other to form an arc-extinguishing space each have a radius of curvature inthe cross section thereof greater than the radius of curvature of the discharge initiating portion of each electrode.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

' FIG. 1 is'a plan view of discharge electrodes forming a discharging gap for the conventional type of lighting arrester devices;

FIG. 2 is across sectional view of one of the electrodes taken along the line 11-11 of FIG. 1;

FIG. 3 is a graph showing a sticking time of electric arc as a function of a pressure of sulfur hexafluoride (SP6);

FIG. 4 is a graph showing a sticking time of electric arc as a function of a discharging gap size for sulfur hexafluoride;

FIG. 5 is a fragmental plan view of discharge electrodes forming a discharging gap for a lighting arrester in accordance with the principles of the invention;

FIG. 6 is an enlarged cross sectional view taken along the line Vl-VI of FIG. 5;

I FIG. 7 is a graph illustrating the relationship between a discharge voltage and a radius of curvature of a cross section of a discharge initiating portion of an electrode disposed in SF gas;

FIG. 8 through 10 are fragmental cross sectional view of modifications of the discharge electrod shown in FIG. 5;

F IG. 11A is a plan view of discharge electrodes forming a discharging gap for a lighting arrester in accordance with the principles of the invention;

FIGS. IllB and C are cross sectional views taken along the lines b-b and c-c of FIG. 11A respectively;

FIG. 12 is a cross sectional view of an embodiment according to another aspect of the invention; and

FIG. 13 is a graph useful in explaining the operation of the embodiment shown in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIGS. 1 and 2 in particular, it is seen that an arrangement disclosed herein comprises a pair of elongated flat electrodes 1 round at their ends and edges and disposed in divergent opposite relationship to form a V-shaped discharging gas high in electrically'negative property such as gaseous suffur hexafluoride (SF Upon the occurrence of an electric surge voltage across the electrodes resulting from a lighting stroke for example, a discharge is initiated across the minimum portion 'of the discharging gap adjacent one end of the discharge electrodes. Even after the surge current has decayed the folldwcurrent flows from the associated ac power source until it is extinguished at a first voltage zero point. In order to prevent thev electrodes from being wore away on account of a long lasting flow current and also improve the interrupting performance, magnetic means'(not shown) is operatively coupled to the discharge electrodes to establish a magnetic field substantially perpendicular to the plane of FIG. 1 torapidly drive electric, arcs due to the follow current toward a broader arc-extinguishing space 3 defined by the other end portions or arc-extinguishing portions of the discharge electrodesl.

Due to its high dielectric strength, the use of sulfur hexafluoride (SF gas causes inevitably a decrease in .size of the gap portion formed between the discharge remarkable particularly when the SF gas is utilized under a high pressure. This is the reason why in SF "gas filled lighting arrester devices, it has been difficult to make the discharge initiating gap size and the gas pressure much large.

With the' minimum; gap size made smaller, a serious problem has beenencountered in magnetically driving From the foregoing it will be appreciated that for lighting arrester devices including the discharging gap structure such as shown in FIG. 1 and utilizing the SP gas as the medium for extinguishing electric arcs, it has been impossible to make both the gas pressure and the gap size large in view of the standpoint of the discharge voltage required for the gap. This has resulted in the I disadvantages that the sticking time of electric arcs can devices is not good.

not reduced and the interrupting performance of the In addition, because of the small gap size in the ar rangement of FIG. 1, a flow of high current through the discharge electrodes 1 has led to the occurrence of the phenomenon that electric arcs due to the high discharge current are suddenly disabled to be magneticallydriven with the result that the arrangement can not bear heavy duties.

The invention contemplates to eliminate the abovementioned disadvantages of the prior art type discharging 'gap structures for lighting arrester devices by rendering of discharge electrodes including the respective discharge initiating portions having a radius of curvature in the cross'section of a discharge initiating porelectric arcs. That is, some interval of timeexpires beforethe arc is initiated to be moved after a magnetic field has been applied to the arcfor that purpose. This is called the sticking phenomenon. The sticking phenomenon strongly appears in mess, gas. Particularly, the sticking time is longer for gases .under lower pressures and small gap sizes. In this connection, it is to be noted that the strength of the driving magnetic field only affects a speed of movement of an electric arc and that an increase in strength of the magnetic field can not lead to a great decrease in sticking time. In order to ensure the interrupting performance of the discharge points or the discharge initiating portions 2, it is required to rapidly initiate the movement of the electric arc toward the arc-extinguishing space 3. It has been found that the stickingtime should be equal to or less than at most 0.8 millisecond. v

FIG. 3 shows a mean sticking time of electric arc in milliseconds as a function of a pressure of SF gas in absolute atmospheres with a gapsize of l millimeter and an arc current of 2 kiloamperes. FIG. 3 depicts that even for 4 absolute atmospheres of the SF, gas the sticking time. is still in the order of 0.4 millisecond with the gap 1, millimeter long.

FIG. 4 shows a mean sticking time in milliseconds as a function of a gap size for SF, gas under 2 absolute atmospheres and'an-arc current of 2 kiloamperes. From FIG. 4, it is seen that for the SF, gas under 2 absolute atmospheres, thesticking time can tion of an electrode not greater than a predetermined small value thereby to increase the discharging gap size without increasing voltage across the gap.

Referring now to FIGS. 5 and 6, wherein like reference numerals designate the components identical to those shown in FIGS. 1 and 2, there is illustrated a pair of discharge electrode constructed in accordance with the principles of the invention. The arrangement is difierent from that shown in FIGS. 1 and 2 only in that in FIGS. 5 and 6 the discharge initiating portion 2 of jacent edge surface of the other electrode-l to form a V-shaped discharging gap therebetween. This measure causes an electric field to be unevenly distributed around the discharge initiating portions 2 of the discharge electrode leading to a decrease in discharge voltage across the gap with the gap size remaining unchanged.

Thus it will be appreciated that in the arrangement as shown in FIGS. 5 and 6, the discharge voltage is able to decrease for larger gap sizes as compared with the prior art type arrangements such as shownin FIGS. land 2. Accordingly, the sticking time of the electric arc becomes shorter for a given gap size thereby to improve the interrupting performance. Further, in addition to decreasing damages to the discharge initiating portions owing to the short sticking time of electric arc, one can increase the upper limit of a current above which electric arcs developed across the discharge in-' itiating portions of the adjacent electrodes are disabled to be magnetically driven. v v

As in the arrangement shown in FIGS. 1 and 2, the electric arcs appearing between the discharge initiating portions 2 are rapidly driven toward the broader end of the discharging gap by the action of a magnetic field established substantially perpendicularly to the plane of FIG. 5 by magnetic means (not shown).

FIGS. 8 through show modifications of the invention. In FIG. 8, the discharge initiating portion 2 includes a pair of main opposite surfaces formed of flat parallel surfaces and a pair of lateral edges of truncated wedge shaped cross section. A circle 4 is shown in FIG. 8 as inscribing three sides of the end'portion of the edge and having a radius of R. In the arrangement of FIG. 8 the circle shown is the largest one that may be inscribed and the radius of the circle4 should be equal to or less than the predetermined small value R. FIG. 9 shows an arrangement similar to that illustrated in FIG. 8 excepting that the lateral edge is formed of a rectangular extension of the main body of the discharge initiating portion thiner than the latter. The thickness of the extenvalue R. An arrangement shown in FIG. 10 is different from that illustrated in FIG. 8 only in that in FIG. 10,

the lateral edge is formed of a pointed wedge having a radius of curvature regarded to be of zero. v

Experiments were conducted with the discharge electrodes according to the invention in an atmosphere of SF gas by varying the pressure P of the gas, the minimum gap size I, and the minimum radius of curvature R of the discharge initiating portion of the electrode. The results of the experiments have indicated that a substantially constant relationship exists between V/Pl and R where V designates the discharge voltage across the minimum gap although it somewhat depends upon the gap structure. FIG. 7 illustrates typically the relationship between V/Pl and R. In FIG. 7, the axis of ordinates represents the V/Pl in kilovolt per atmosphere-centimeter and I the axis of abscissas represents the RH. From FIG. 7 it is seen that if the RH is less than the order of 0.5 that the WW abruptly decreases and that a value of the RH not greater than 0.3 permits the minimum gap size I to be fairly large for the same discharge voltage.

In the conventional lighting arrester devices utilizing nitrogen as the medium for extinguishing electric arcs,

' the R/lhas been selected to be large because the discharge voltage is scarcely different between larger and smaller values of the RI! while the nitrogen is less in dielectric strength than the SP gas.

As will readily be understood from the graph shown in FIG. 7, a decrease in R/! permits a discharge voltage for a given gap size to be very less than that for the same gap size having a larger value of the RH. On the other hand, the ability of the gap to restore the electrically insulating property thereacross after the interruption of the particular discharge current is substantially determined by the minimum gap size l but not much affected by the R/! which has been verified by experiments. Therefore a ratio between the dielectric strength of the discharging gap after the interruption of a discharge current and the discharge voltage can be made sufficiently large to greatly improve the performance of the arresters gap which is, in turn, determined by that ratio.

As previously described, an increase in gap size permits the sticking time to decrease. For a given discharge voltage, the gap size canincrease by rendering the cross sectional profile of the electrode more sharp. Therefore the cross sectional profile of the electrode can be made sharp to realize a decrease in sticking time. It has been found that aratio between the radius of curvature R of the discharge initiating electrode portion and the discharge initiating gap size I or R/! is preferably equal to or less than 0.3 with satisfactory results.

If the radius of curvature R remains unchanged from one end to the other end of the electrode, the value of R/l becomes very small because the gap size becomes larger in an arc-extinguishing space at the other end of the electrode. This causes an arc length in the arcextinguishing space such as the space 3 shown in FIG. 1 to be fairly large in order to ensure the high dielectric restoring property of that space after the follow current has long lasted resulting in the following disadvantage:

Assuming that a rectangular current flows through a lighting arrester device, an electric energy developed across the associated discharging gap is equal to the product of the magnitude and duration of current multipled by a voltage across both ends of an electric are generated across the gap or an arc voltage. Accordingly if the electrode has the radius of curvature remaining unchanged from one to the other end thereof and the gap size in the arc extinguishingspace is fairly large then the arc voltage is correspondingly raised to increase an electric energy consumed within the discharging gap. This does not permit the interrupting performance to be much improved.

FIG. 11 shows another modification of the invention contemplating to eliminate the disadvantage just described. The arrangement illustrated comprises a pair of discharge electrodes disposed in the same manner as above described in conjunction with FIGS. 1 and 2 and substantially similar in shape to those shown in FIGS. 5 and 6. The electrodes 1 are connected to the terminals 5 respectively for external connection. As best shown in FIG. 11 B, the electrode 1 includes a pair of main opposite flat surfaces disposed in parallel relationship and one end portion or the discharge initiating portion 2 thereof includes that lateral edge facing the corresponding edge of the other electrode 1 which edge has a pointed wedge-shaped cross section. If desired the discharge initiating portion may have any one of those above described in conjunction with FIGS. 6, 8, 9 andl0. Then a ratio between a radius of curvature R in the cross section of that lateral edge and the size of the associated gap I or R]! is equal to or less than 0.3. This measure serves to render the sticking time sufficiently short upon magnetically driving an electric are appearing across the gap.

Thenthe electric arc is magnetically driven until it reaches the arc-extinguishing space 3 disposed at the other ends of both electrodes 1. As best shown in FIG. 11 C,,that portion of the electrode 1 disposed in the arc extinguishing space 3 has its lateral edge facing the corresponding edge of the other electrode 1 which has a circular arc-shaped cross section for the purpose of rendering the radius of curvature of the cross section of the edge relatively large. Therefore the arc-extinguishing space 3 become high in discharge voltage which permits the separation between the electrodes in the arc-extinguishing space to be relatively small without decreasing the restoring rate of the dielectric strength of that space and which causes an electric energy consumed within that space to be as low as possible. The opposed lateral edges of the electrodes gradually increases in radius of curvature from its minimum value on the discharge initiating portion at one end to its maximum value at the other ends.

Therefore the arrangement of FIG 11 providesa discharging gap structure for a lighting arrester device A excellent in interrupting performance because the sticking timeis sufficiently short and an electric energy consumed therein is minimized.

According to another aspect, the invention provides means for early restoring a dielectric strength of a discharging'gap after the interruption of a discharge current, .which will be subsequently described in conjunction with FIG. 12. In FIG. 12 a pair of discharge electrodes 1 are centrally disposed inan arc-extinguishing compartment formed of a pair of arc-extinguishing plates 6 and 7 disposed in opposite parallel relationship and connected together through a pair of side plates simulating the arc-extinguishing space 3'as shown in FIG. 11. When an electric are 8 has appeared across the electrodes 1, the arc-extinguishing plates 6 an'd'7 are fused or evaporated by means of a thermal energy generated by the arc. At that time'the plates 6v and 7 ab-' sorb a heat of fusion or evaporation to cool the electricarc. Also a quantity of heat generated between the arc- I extinguishing plates is slowly dissipated into the exterior vforasufficiently long interval of time. In this way, thedielectric strength of the discharginggap across the electrodes-1 isrestored as early as possible. a

ace extending up to millimetersmeasured from s the central plane of theelectrodes with an electrically negative gas such as SF gas in an amount sufficientto absorb a quantity of heat generated by electric arcs appearing across the electrodes. While the abovementioned distance d somewhat depends upon both the pressure of the electrically negative gas and a speed of arc movement it has be found that the distance d can be effectively up to 15 millimeters in so far as the pressure of the gas and the speed of movement of arc range in the practical limits.

On the other hand, an energy of 1.6 X 10 joules is required to raise 1 gram of SE, gas from room temperature to a temperature of 3,000 K required for the gas to be thermally ionized. Also assuming that for all practical purposes of lighting arresters a discharge current is of 4,000 amperes with a duration of 2 milliseconds and an arc voltage is of 200 volts, an energy consumed inthe discharging gas amounts at 1.6 X 10 joules. Thereforeabout 0.1 gram of SF.; gas is sufficient to effe'ctively consume the energy due to the electric arcs while. preventing the thermal ionization from occur- I-Iowever, with a high discharge current flowing through the electrodes, the resulting electric arc generates a large quantity'of heat. Accordingly the arcexting uishing plates 6 and 7 can not sufficiently cool the'arc whereby that space surrounding the are greater increases in temperature leading to an unsatisfactory arc-extinguishment. If a spacing d between the common central plane of the electrodes 1 and the inner wall surface of each plates 6 and 7 is narrow as in the prior art practice then the arc-extinguishing compartment can not filled with a sufficient quantity of SF gas excellent in. arc-extinguishing performance. This does not permita time constant for restoring the dielectric strength to be of a satisfactorily small value. 3 i

The arrangement of FIG. '12 is characterlzediin that the distance between the'common central plane of the electrodes 1 and the inner wall surface of. each arcextinguishing plate 6 or 7 is;sufficiently great while the arc-extinguishing compartment is filledwith an electrically negative gas such as SF gas in addition to the particular configuration of the electrodes as above described in conjunction with FIG. 11.

FIG. 13 illustrates the result of experiments conducted With the arrangement of FIG. 12 in order to determine the relationship between the time constant Y for restoring the dielectric strength and the distance d as above described. From FIG. 13 wherein the axis of ordinates represents the time'consta'nt in milliseconds and the axis of abscissas represents the distance d in millimeters, it is seen that for the SF gas under 4 absolute atmospheres the time constant rapidly decreases ring. I r

What we claim is: '1. A discharge gap structure for use in a lightning arrester comprising: means defining an enclosed envelope; a pair .of. elongated discharge electrodes disposed-in opposed spaced-apart relationship within,

during use of the discharge gap structure to extinguish electric arcs developed across said gap between said pair of discharge electrodes. 7

.2. A discharge gap structure according to claim 1; wherein said pair of discharge electrodes are disposed in a generally -V-shaped arrangement with said discharge initiating portions forming the base portion of the V and being the portions of said discharge electrodes which are nearest to each other. i 1

3. A discharge gap structure accordingto claim 2; wherein each said discharge electrode has an arc-extinguishing portion forming one of the diverging endsof the V and has a cross-sectional profile sized such that the largest possible circle which may be inscribed therein has a radius of curvature greater than that of the circle inscribed in said discharge initiating portion of the same discharge electrode.

4. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of a circular segment. I

5. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of a trapezoid.

6.:A discharge gap structure according to claim 1; wherein each 'said discharge initiating portion hasthe cross-sectional profile of a rectangle.

7. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of 'a pointed taper. 

1. A discharge gap structure for use in a lightning arrester comprising: means defining an enclosed envelope; a pair of elongated discharge electrodes disposed in opposed spaced-apart relationship within said envelope defining therebetween a gap, each said discharge electrode including a discharge initiating portion having a cross-sectional profile sized such that the largest possible circle which may be inscribed therein has a radius of curvature less than the length of said gap defined by the discharge initiating portions of said pair of discharge electrodes; and an electrically negative gas confined within said envelope effective during use of the discharge gap structure to extinguish electric arcs developed across said gap between said pair of discharge electrodes.
 2. A discharge gap structure according to claim 1; wherein said pair of discharge electrodes are disposed in a generally V-shaped arrangement with said discharge initiating portions forming the base portion of the V and being the portions of said discharge electrodes which are nearest to each other.
 3. A discharge gap structure according to claim 2; wherein each said discharge electrode has an arc-extinguishing portion forming one of the diverging ends of the V and has a cross-sectional profile sized such that the largest possible circle which may be inScribed therein has a radius of curvature greater than that of the circle inscribed in said discharge initiating portion of the same discharge electrode.
 4. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of a circular segment.
 5. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of a trapezoid.
 6. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of a rectangle.
 7. A discharge gap structure according to claim 1; wherein each said discharge initiating portion has the cross-sectional profile of a pointed taper. 